Method and apparatus for producing titanium dioxide

ABSTRACT

A process for reacting titanium tetrachloride vapors with oxygen to produce titanium dioxide wherein the oxygen is introduced into the reactor in at least two points. The process has the ability to control properties, such as particle size, of the raw pigment produced. The temperature of the oxygen introduced to the reactor at the further inlet point is above, below, or at the same temperature of the oxygen introduced at the first inlet point. The further inlet point can be located before or after the all the titanium tetrachloride has been introduced into the reactor. The titanium tetrachloride is introduced at a relatively low temperature, below about 427° C., and the reaction temperature in the reactor is at least about 700° C. The process includes the use of an auxiliary fuel such as carbon monoxide, methane, propane, butane, pentane, hexane, benzene, xylene, toluene, or combinations thereof for increasing the temperature in the reactor. Still further, the process for producing titanium dioxide provides for the addition of aluminum chloride to the reactor. Advantageously, the process allows the pressure in the reactor to be above atmospheric pressure and range between about 0.15 MPa and 4.0 MPa above atmospheric pressure during the production of titanium dioxide. A reactor is also provided with an aluminum chloride generator for heating the titanium tetrachloride and delivering aluminum chloride to the reactor.

This application is a continuation-in-part of application Ser. No.08/687,280, filed Jul. 25, 1996, now U.S. Pat. No. 5,840,112.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing titaniumdioxide by reacting titanium tetrachloride vapors with oxygen and to animproved reactor for use in such a system. The process and reactor ofthe present invention provides the ability to control properties, suchas particle size, of the titanium dioxide product.

It is well-known that titanium tetrachloride reacts with oxygen in thevapor phase to form titanium dioxide and that this reaction is initiatedby heating the reactants to a suitable temperature. However hot titaniumtetrachloride is highly corrosive and therefore many useful materials ofconstruction for heat exchangers used to heat titanium tetrachloride arerapidly corroded. In practice this generally imposes an upper limit ofabout 400° C. (752° F.) on the temperature to which titaniumtetrachloride can be heated by conventional heat exchangers.

A suitable temperature for the reactants (oxygen and titaniumtetrachloride) is about 950° C. (1742° F.) and, in order to achieve thistemperature in known processes, the oxygen feed must be heatedsufficiently to compensate for the above-mentioned relatively lowtitanium tetrachloride temperature. Frequently, oxygen is heateddirectly or heated by an electrical discharge to temperatures of about1427-1871° C. (2600-3400° F.) as oxygen is introduced into the oxidationreactor in combination with an auxiliary fuel. The use of these methodsintroduce unwanted impurities such as, for example, carbonaceousresidues from the fuel or metallic impurities from the electrodes usedfor the electrical discharge.

SUMMARY OF THE INVENTION

According to the invention a process for the production of titaniumdioxide comprises reacting titanium tetrachloride with oxygen at apressure above atmospheric pressure and at a reaction temperature of atleast about 700° C. (1292° F.) in an oxidation reactor, the oxygen beingintroduced into the reactor at a first inlet point and at least onefurther inlet point. Optionally, the titanium tetrachloride may beintroduced as a mixture with aluminum chloride and heated to atemperature of at least about 350° C. (662° F.), the aluminum chloridebeing formed by reaction of aluminum and chlorine and the heat generatedby this reaction being used to heat the titanium tetrachloride. Thealuminum chloride may also be added by dissolving the aluminum chloridein the titanium tetrachloride.

According to the present invention, a reactor for producing titaniumdioxide by reacting titanium tetrachloride vapors with oxygen comprisesa means for forming a first reaction zone and an oxidizing gasintroduction assembly for receiving oxygen at a predeterminedtemperature level and passing oxygen into the first reaction zone. Theoxidizing gas introduction assembly comprises a conduit having anupstream and a downstream end and an opening extending therethroughintersecting the upstream and the downstream ends where oxygen ispassable through the opening in the conduit for passing into the firstreaction zone. The reactor further comprises a first titaniumtetrachloride introduction assembly for receiving titanium tetrachloridevapors at a first predetermined temperature and passing titaniumtetrachloride vapors into the first reaction zone for reacting withoxygen to produce a mixture including titanium dioxide. Still further,the reactor comprises a means for passing the titanium tetrachloridevapor at the predetermined temperature into the first reaction zone andan including means for forming a second reaction zone spaced a distancedownstream from the first reaction zone. The reactor also comprises asecond oxidizing gas introduction assembly for receiving oxygen at asecond predetermined temperature and passing the oxygen at the secondtemperature into the second reaction zone for reaction with titaniumtetrachloride in the mixture from the first reaction zone to produce amixture including titanium dioxide, the reaction of oxygen at the secondtemperature with the mixture passed from the first reaction zonereducing the volume of oxygen at the first temperature level requiredfor a given volume of titanium dioxide produced and a means for passingthe oxygen at the second temperature into the second reaction zone.Still further, the reactor comprises an aluminum chloride generator forheating the titanium tetrachloride vapors to a first predeterminedtemperature and a flowline for passing titanium tetrachloride from thealuminum chloride generator to the titanium tetrachloride introductionassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of the equipment for preheating oxygen forintroduction into the reaction zones in the reactor.

FIG. 2 is a graph showing the relationship of CBU and tint tone vs.TiCl₄ to O₂ ratio at the primary TiCl₄ slot.

FIG. 3 is a graph showing tint tone vs. consistency.

FIG. 4 is a diagrammatic view of one embodiment of the system of thepresent invention showing the relative positions of the second TiCl₄introduction assembly and the second O₂ introduction assembly on thereactor.

FIG. 5 is a diagrammatic view, similar to FIG. 4, showing anotherembodiment of the present invention.

FIG. 6 is a diagrammatic view, similar to FIG. 4, showing anotherembodiment of the present invention.

FIG. 7 is a diagrammatic view, similar to FIG. 4, showing anotherembodiment of the present invention.

FIG. 8 is a diagrammatic view, similar to FIG. 4, showing anotherembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Titanium dioxide (TiO₂), which is useful as a pigment, is produced on acommercial scale by reacting titanium tetrachloride vapor (TiCl₄) withoxygen (O₂). In one commercial process, a preheated oxidizing gas ispassed into a reaction zone and preheated titanium tetrachloride vaporis passed into the same reaction zone where the titanium tetrachloridevapor is reacted with the oxygen contained in the oxidizing gasaccording to the following reaction:TiCl₄+O₂→TiO₂+2Cl₂

In such a prior art process the combined temperature of the reactants(titanium tetrachloride and oxygen), before reaction, had to be at leastabout 871° C. (1600° F.) in order to sustain the oxidation reaction and,preferably, the combined temperature of the reactants was between about899° C. (1650° F.) and about 982° C. (1800° F.). In one process, theoxidizing gas was preheated for introduction into the reaction zone to atemperature of about 982° C. (1800° F.) and titanium tetrachloride vaporwas preheated for introduction into the reaction zone to a temperatureof about 954° C. (1750° F.).

Titanium tetrachloride vapors at relatively high temperatures of about954° C. (1750° F.) are highly corrosive. Operation at such a hightemperature requires frequent maintenance and repair of the titaniumtetrachloride preheating equipment. It is therefore desirable to developa system for producing titanium dioxide by reacting titaniumtetrachloride vapor with oxygen utilizing titanium tetrachloride vaporspreheated to minimum temperature levels, such as below about 204° C.(400° F.) since this would minimize the cost of repair and maintenanceof the titanium tetrachloride preheating equipment.

A reactor of the type utilized in the process for producing titaniumdioxide by reacting titanium tetrachloride vapor with oxygen asdescribed above was disclosed in U.S. Pat. No. 3,512,219, issued toStem, and the configuration with a dual slot oxidizer (DSO) in U.S. Pat.No. 4,803,056 issued to Morris, et al., both specifically incorporatedherein by reference.

In this prior process, pure oxygen was heated in a metal alloy tubefurnace. In one embodiment, oxygen could only be heated to a maximumtemperature of about 982° C. (1800° F.) due primarily to the thermalefficiency and the materials of construction of the oxygen preheatingapparatus. Thus, in this process, titanium tetrachloride vapors also hadto be heated to a temperature of about 982° C. (1800° F.) in thetitanium tetrachloride vapor preheating apparatus. In the alternative,additional oxygen preheating equipment might be added to the existingoxygen preheating equipment in an effort to elevate the oxygentemperature to a level above 982° C. (1800° F.), thereby permitting theutilization of titanium tetrachloride vapors which have been preheatedto lower temperature levels, below 982° C. (1800° F.). However, theadditional oxygen preheating equipment represents a substantial expensewhich might not be offset by any savings in the titanium tetrachloridevapor preheating apparatus resulting from the lower temperaturerequirements for the titanium tetrachloride vapors.

In the above process, the titanium tetrachloride vapor preheatingequipment utilized silica pipe for the containment of the highlycorrosive titanium tetrachloride vapors. The size of the silica pipe waslimited to a maximum of about six inches because of manufacturingtechniques suitable for producing a relatively flawless silica pipe.Also, the strength and integrity of welded silica pipe joints decreasewith increasing diameters and breakage is more probable with higherdiameter silica pipes. A primary problem with silica is the failurerate. The failure rate is proportional to the surface area of the silicapipe. As the area of the silica pipe increases, the failure rateincreases. Further, the maximum permissible pressures within the silicapipe decreases with increasing diameters and above six inch diametersilica pipes might result in working pressures insufficient toefficiently drive the titanium tetrachloride vapors downstream from thetitanium tetrachloride vapor preheating equipment.

Auxiliary fuel normally is added at the upstream end of the reactor nearthe oxidizing gas introduction assembly. Injection of auxiliary fuels,such as carbon monoxide and methane directly into the reactor tostabilize the flame in the reactor has been suggested as a means forlowering the temperature level requirements for the titaniumtetrachloride vapors, thereby increasing the capacity of existingtitanium tetrachloride vapor preheating equipment, i.e., the silica pipepreheaters. This approach can lead to a reduction in temperature forpreheating the TiCl₄ from about 954° C. (1750° F.) to about 399° C.(750° F.) when using supported combustion from an auxiliary fuel.However, using supported combustion generates combustion products whichdilute the chlorine recycle gas and result in larger capacity downstreamequipment being required to process the increased gas load.

The present invention determined that properties, such as particle sizeand other related properties, of the raw pigment produced in oxidationcan be controlled over a wide range by controlling the titaniumtetrachloride to oxygen ratio in the zone of the reactor where particlesinitially start to form or are nucleated. According to the presentinvention, the properties of raw pigment can be controlled by changingthe ratio of TiCl₄ to O₂ in the region of the reactor where the TiO₂particles start to form or are nucleated. Controlling the ratio of TiCl₄to O₂ by this method requires a second O₂ addition downstream in thereactor to meet stoichiometric requirements for the over all reaction.Similar control of particle properties can be achieved by varying themixing rate or injection angles, but these controls cannot be asconveniently adjusted as the flow rates of the TiCl₄ and O₂ reactants.

Tests performed using a hot secondary oxygen flow that was split usingorifice plates produced pigments with much more positive tint tones, butsince the relative oxygen flows were controlled by orifice plates it wasdifficult to control each O₂ flow so as to control particle size. Onetest performed regulated the oxygen flows while the oxygen was stillcold and then heated each stream to the desired temperature. This testallowed for independent control of the volume and temperature of eachgas stream. The use of secondary oxygen can be used to increase tinttone, scatter, and reduce aggregation. Reducing aggregation results indecreasing consistency, oil adsorption, dispersant demand for thefinished pigments. A pigment with a more positive tint tone can beproduced by using secondary oxygen. Diverting some of the oxygen goingto the front of the oxidizer to a position behind the first TiCl₄ slothave made finished pigments with acrylic tint tones as positive as about−3.2. It is expected that tint tones more positive than −3.2 areobtainable using a secondary oxygen slot.

Shown in FIG. 1 is a schematic for the primary and secondary O₂ flowsconstructed in accordance with the present invention for use in aprocess for producing titanium dioxide by vapor-phase oxidation oftitanium tetrachloride. In general, the reactor 10 comprises: a firstoxidizing gas introduction assembly 12 which is adapted to receiveoxygen from oxygen preheat equipment 14 by way of a flowline 16 and passthe oxygen at a first predetermined temperature into the first reactionzone 18 formed in the reactor 10; a first titanium tetrachloride vaporintroduction assembly 20 which is adapted to receive titaniumtetrachloride vapor at a first predetermined temperature from titaniumtetrachloride preheat equipment by way of a flowline 24 and to pass thetitanium tetrachloride vapor at the first predetermined temperature intothe first reaction zone 18; and a second oxidizing gas introductionassembly 26 which is adapted to receive oxygen at a second predeterminedtemperature, which can be above, below, or the same temperature as thefirst oxygen temperature, from second oxidizing gas preheat equipment 28by way of a flowline 30 and to pass oxygen at the second predeterminedtemperature into the second reaction zone 32, the mixture from the firstreaction zone being passed into the second reaction zone for reactingwith oxygen at the second temperature which simultaneously is beingpassed into the second reaction zone.

A second addition of titanium tetrachloride may be introduced into thereactor through a second titanium tetrachloride introduction assembly34. The second titanium tetrachloride introduction assembly 34 is spaceda distance from the first titanium tetrachloride introduction assembly20. The second titanium tetrachloride introduction assembly 34 receivestitanium tetrachloride vapors at an elevated temperature and passes thetitanium tetrachloride vapors into the reactor near the second reactionzone 32. The second oxidizing gas introduction assembly 26 can belocated between the first and second titanium tetrachloride introductionassemblies 20 and 34. Alternatively, the second oxidizing gasintroduction assembly 26 can be located after the second titaniumtetrachloride introduction assembly 34 such that the second titaniumtetrachloride introduction assembly is between the first titaniumtetrachloride introduction assembly and the second oxidizing gasintroduction assembly.

The reactor is a continuous tube but can be divided into two zones forpurposes of discussion. As used herein “first reaction zone” refers tothe region of the reactor near the first oxygen inlet point where thereaction between TiCl₄ and O₂ is initiated and where the TiO₂ particlesare nucleated. As used herein, “second reaction zone” refers to theregion of the reactor extending downstream from the first reaction zonewhere interparticle reactions occur and the particles grow by theaerosol process to the desired size. The second titanium tetrachlorideintroduction assembly is positioned on the reactor such that it islocated within the second reaction zone. It is believed that thereaction between titanium tetrachloride and oxygen occurs throughout thereactor and is not isolated in any one particular zone.

In a preferred embodiment, oxygen is fed to the reactor 10 from the O₂header shown at the bottom of FIG. 1. Oxygen preheaters 14 and 28receive oxygen from the header and are capable of preheating oxygen atabout 954° C. (1750° F.). The preheaters 14 and 28 heat the oxygen tothe respective predetermined temperatures. Oxygen preheater 14 heatsfrom about 50% to about 95% of the total O₂ to be fed into the reactorand preheater 28 heats the balance of the total O₂, from about 5% toabout 50%, to be fed into the reactor 10. The primary oxygen leavespreheater 14 through an insulated pipe 16 that coaxially joins thelarger tube that serves as the reactor at the oxidation gas introductionassembly 12. An inlet for auxiliary fuel and scour media is located nearthe oxidation introduction assembly 12 and serves to introduce the fuelto the hot oxygen and to direct scour media for cleaning the reactorwalls in the reactor. The inlet is located far enough upstream in thereactor to allow for nearly complete combustion of the auxiliary fueland to provide the proper trajectory for the scour media entering thereactor. The secondary oxygen leaves preheater 28 through an insulatedpipe 30 and enters the reactor at the second oxidizing gas introductionassembly 26.

The first increment of TiCl₄ which has been preheated to about 399° C.(750° F.), primary TiCl₄, is introduced into the reactor through thefirst titanium tetrachloride introduction assembly 20. The hot primaryO₂ and TiCl₄ are swept into the first reaction zone 18 of the reactor.It will be appreciated that properties of the pigment including tinttone can be accurately controlled by varying the relative amounts ofprimary TiCl₄ and primary O₂ fed through oxidizing into the firstreaction zone 18. The amount of TiCl₄ fed through the titaniumtetrachloride introduction assembly 20 has, in practice, varied fromabout two thirds to all of the TiCl₄ fed to the reactor. The hot gasesconsisting of unreacted O₂ and TiCl₄ and very fine TiO₂ particles passfrom the first section of the reactor 18 to the second section of thereactor 25. The remainder of the TiCl₄ is fed through the secondtitanium tetrachloride introduction assembly 34 into the second reactionzone 32 where the TiO₂ particles are grown to full size.

The amount of TiCl₄ that can be fed through the second titaniumtetrachloride introduction assembly 34, secondary TiCl₄, is determinedby the overall response of the reactor. If too much TiCl₄ is fed throughthe second titanium tetrachloride introduction assembly 34, unreactedTiCl₄ will leave the second reaction zone 32 and appear in the finalproduct. If too little TiCl₄ is added through the second titaniumtetrachloride introduction assembly 34, the consumption of auxiliaryfuel increases. The optimum amount covers a fairly wide range of flowsand is determined by other operating parameters for the reactor. Theamount of secondary O₂ added at the second oxidizing gas introductionassembly 26 is determined by how much unreacted TiCl₄ is present in themixture downstream of the second titanium tetrachloride introductionassembly 34. Typical operating practice is to add enough total O₂ sothat the export gases contain from about 7 to 10 percent O₂.

Preferably, oxygen preheat equipment 14 is constructed to heat theprimary oxygen to a temperature of about 954° C. (1750° F.),advantageously from about 815° C. (1500° F.) to about 982° C. (1800°F.). The second oxygen preheat equipment 28 advantageously heats thesecondary oxygen from about 25° C. (77° F.) to temperatures as high asabout 1038° C. (1900° F.). Such oxygen preheat equipment is commerciallyavailable and is well known in the art.

In a preferred embodiment, titanium tetrachloride preheat equipmentheats titanium tetrachloride to a temperature of about 177° C. (350° F.)to produce titanium tetrachloride vapors. Such titanium tetrachloridepreheat equipment is commercially available and is well known in theart. In one embodiment, for example, the titanium tetrachloride isheated and vaporized in a shell-and-tube type heat exchanger operatingat a temperature of about 177° C. (350° F.). One type of heater is ashell-and-tube heat exchanger with a u-shaped tube bundle of nickel andglass-lined carbon steel sheet. The tube-side heating medium normally issteam, but may, at temperatures approaching 204° C. (400° F.), be someother heat transfer fluid such as Dow-therm, should suitable steampressure be unavailable. One silica pipe heater which is useful forreceiving titanium tetrachloride at about 204° C. (400° F.), is atubular radiant-heat furnace with vertical silica pipe. The titaniumtetrachloride vapors introduced into the reactor through the firsttitanium tetrachloride introduction assembly 20 are further heated to atemperature of less than about 427° C. (800° F.), preferably less thanabout 399° C. (750° F.), before injection into the reactor. The titaniumtetrachloride vapors introduced through the second titaniumtetrachloride introduction assembly 34 are preferably introduced at atemperature of about 177° C. (350° F.). Preferably, one titaniumtetrachloride preheater is used to preheat the TiCl₄ to produce theTiCl₄ vapors. The preheated TiCl₄ vapors would then be split into twolines, one directed to the second titanium tetrachloride introductionassembly and the other to additional heating equipment for furtherheating before being passed to the first titanium tetrachlorideintroduction assembly.

In a preferred embodiment, assuming a capacity of 100 tons pertwenty-four hour period of titanium dioxide produced utilizing reactor10, the flow or primary oxygen gas into the oxidizing gas introductionassembly and through the reactor 10 is about 60 pound mole per hour, theflow level of primary titanium tetrachloride into the titaniumtetrachloride introduction assembly 20 and through the reactor 10 isabout 104 pound mole per hour and the flow of secondary oxygen at thesecond temperature into the second oxidizing gas introduction assemblyand through reactor 10 is about 60 pound mole per hour. In thisembodiment, about one pound mole per hour of oxygen together with twohundred pounds per hour of sand are passed through the injection tube.It will be appreciated that secondary oxygen could be used with thereactor of the present invention without the use of scour sand in thereaction zone.

In operation, oxygen is preheated in oxygen preheat equipment 14 to thepredetermined temperature and then passed at a controlled, predeterminedrate through flowline 16 to the oxidizing gas introduction assembly 12and passes into the first reaction zone 18.

Titanium tetrachloride is preheated in titanium tetrachloride preheatequipment to a predetermined temperature and passed through flowline 24at a controlled rate into titanium tetrachloride introduction assembly20 and into the first reaction zone 18, where oxygen at the firsttemperature and titanium tetrachloride react to produce a mixtureincluding particles of titanium dioxide, this mixture being passeddownstream into the second reaction zone 32. Oxygen is preheated insecond oxidizing gas preheat equipment 28 to predetermined secondtemperature and passed at a controlled rate through flowline 30 to thesecond oxidizing gas introduction assembly 26 and into the secondreaction zone 32, where oxygen at the second temperature reacts with thetitanium tetrachloride in the mixture passed from the first reactionzone 18 to produce a mixture including additional titanium dioxide, themixture from the second reaction zone 32 being passed downstream forfurther processing in a manner known in the art of producing titaniumdioxide by vapor phase oxidation of titanium tetrachloride.

In order to ensure rutile is the dominant phase in the titanium dioxideproduct, the temperature in the reaction zones must be above a minimumtemperature level of about 1204° C. (2200° F.). Reagents, such asaluminum chloride and water vapor, may be added to the reactor forcontrolling or modifying titanium dioxide pigment properties. Becausealumina and water act as rutilization agents, the minimum temperaturelevel depends on the amount of alumina and water present in the system.As the water and alumina content increases, the rate of rutilizationincreases.

The combined temperature of the reactants, prior to reaction, to producethe required reactions, must be at least 871° C. (1600° F.) to sustainthe oxidation reaction and preferably, the combined temperature of thereactants, before reaction, should be in the range of from about 899° C.(1650° F.) to about 982° C. (1800° F.). In one operational process forproducing titanium dioxide by vapor-phase oxidation of titaniumtetrachloride, oxygen is preheated to a temperature level of about 982°C. (1800° F.) and titanium tetrachloride is preheated to a temperaturelevel of above about 954° C. (1750° F.). In this process, oxygen andtitanium tetrachloride vapors are reacted in a reaction zone utilizing areactor like that disclosed in Stem, et al., U.S. Pat. No. 3,512,219, toproduce a mixture including some titanium dioxide, and the mixtureconsisting of unreacted TiCl₄ and O₂ and reaction products is passeddownstream for further processing.

The reaction of titanium tetrachloride vapors with oxygen to formtitanium dioxide is exothermic. In a completely adiabatic system,starting with 177° C. (350° F.) TiCl₄ vapor and 25° C. (77° F.) oxygen,a reaction temperature of about 1316° C. (2400° F.) is attainable, whichis above the minimum temperature of 1204° C. (2200° F.) required toinsure rutile as the dominant phase in the titanium dioxide product ofreaction. The system of the present invention utilizes this heat ofreaction to reduce the preheat requirement for a portion of the titaniumtetrachloride vapors utilized.

Utilizing only the first reaction zone and assuming a flow of oxygenfrom oxygen preheat assembly of 60 pound moles per hour at a temperaturelevel of about 982° C. (1800° F.) and assuming a flow of titaniumtetrachloride from the titanium tetrachloride preheat assembly of 52pound moles per hour at a temperature of about 982° C. (1800° F.), about4150 pounds per hour of titanium dioxide are produced and the heat ofreaction in the first reaction zone, assuming a completely adiabaticsystem will generate a temperature of above 1316° C. (2400° F.).

In one embodiment, the walls of reactor 10 are cooled (fluid cooling) toprotect the walls and to keep the titanium dioxide product fromsintering on the walls of the reactor such that a scouring media may beused to remove the titanium dioxide. The walls of the reactor maybecooled by providing a purge of nitrogen or chlorine gas through thereactor walls.

The possibility of controlling raw pigment properties using TiCl₄concentration was tested using the oxidizer configuration shown in FIG.1. The properties of the pigment produced from a raw pigment can beestimated by measuring the Carbon Black Undertone (CBU) of the rawpigment. To measure CBU, a sample of the raw pigment and a standardsample are each mixed in a paste with carbon black. Reflectancemeasurements are made with a Hunterlab color difference meter, such asModel D25-9. Undertone is calculated from these measurements. The CBUvalue gives a measure of mean particle size of the pigment since thereflected light will change from blue, through the spectrum, to red asthe particle size increases.

An oxidizer was designed so that the ratio of TiCl₄ to O₂ could becontrolled by changing the rate of flow of oxygen in the front of theoxidizer. FIG. 2 is a plot showing how raw pigment CBU and finishedpigment alkyd tint tone could be controlled by controlling the ratio ofTiCl₄ to O₂ added at the front of the oxidizer. It is necessary toalways provide enough O₂ to react completely with the TiCl₄ vapor in thereactor, so, a second addition of O₂ may be necessary. Consistent withthe patent of Morris, the oxidizer may also have one or more TiCl₄injection slots. The significant discovery was that an importantvariable in controlling pigment size was the ratio of TiCl₄ to O₂ in theregion where nucleation is occurring. The data shown in FIG. 2 wascollected with three different configurations of the oxidizer. Thedifferent positions for addition of the oxygen required to oxidize allof the TiCl₄ are shown in FIG. 3. The CBU of the raw pigment, ameasurement of particle size, within the uncertainty of measuringreactant volumes and CBU appears to be largely dependent on the ratio ofTiCl₄ in the region of the oxidizer where nucleation occurs. Theproperties of the finished pigments are also affected by varying theratio of TiCl₄ to O₂. The alkyd tint tone of the finished pigment isshown on the right-hand side of FIG. 2 and the consistency is shown as afunction of tint tone in FIG. 3. The consistencies in FIG. 3 weremeasured after the pigments had been treated with a standard grindingand finishing procedure.

Further inlet points may be positioned such that oxygen may be added tothe reaction stream at a point where any previously added titaniumtetrachloride has not been substantially completely oxidized. Thisenables the oxygen which is added at the further inlet points to be at alower temperature than that added at the first inlet point since thetemperature necessary to initiate reaction is provided by the heat ofreaction of the previously added titanium tetrachloride. The temperatureof the secondary oxygen determines the amount of oxygen that can be usedbefore observing a titanium tetrachloride slip, that is, where unreactedtitanium tetrachloride begins to appear in the titanium dioxide product.By varying the temperature of the secondary oxygen, a wide range of O₂may be added to the reactor thus allowing for control of the particlesize of the titanium dioxide product.

Oxygen is introduced into the reactor as an oxidizing gas stream whichmay comprise a gas containing a relatively low proportion of oxygen suchas air but may also be substantially pure oxygen or another gas mixturesuch as oxygen-enriched air.

The primary oxidizing gas stream is usually preheated beforeintroduction into the reactor to a temperature between about 815° C.(1500° F.) and about 982° C. (1800° F.), preferably between about 899°C. (1650° F.) and about 954° C. (1750° F.). Any suitable means can beused to achieve this temperature but the gas stream is convenientlyheated by passing it through a hollow metal coil which is externallyheated by a gas flame.

Titanium tetrachloride is introduced into the reactor at a temperatureof at least about 149° C. (300° F.), preferably between about 149° C.(300° F.) and about 427° C. (800° F.). This temperature may be achieved,at least in part, by utilizing the heat of reaction of aluminum andchlorine which form aluminum chloride with which the titaniumtetrachloride is admixed. Advantageously, titanium tetrachloride isfirst vaporized in preheating equipment to produce titaniumtetrachloride vapors. Next, the vapors are preheated to about 350° C.(662° F.) to 400° C. (752° F.) by passing through a hollow coil formedfrom a metal such as Inconel which is externally heated by a gas flame,and subsequently passed to an aluminum chloride generator where thevapors are mixed with aluminum chloride and further heated to the chosenreaction temperature usually less than about 427° C. (800° F.). An AlCl₃generator may be provided for one or more of the TiCl₄ inlet points orone common AlCl₃ generator may be used for some or all of the TiCl₄inlet points.

A number of types of aluminum chloride generators are well known in theart and can be used in the process of the invention. For example,powdered aluminum with or without an inert particulate material can befluidized in a reactor by the upward passage of reactant chlorine and/oran inert gas. Alternatively, aluminum can be introduced into a stream ofchlorine gas in particulate form but not necessarily sufficiently finelydivided to fluidize in the gas stream. A fixed bed of particulatealuminum can also be chlorinated by passing chlorine into the bedthrough numerous nozzles surrounding the bed. Other methods includepassing chlorine over molten aluminum or feeding two lengths of aluminumwire into a reactor in which they serve as consumable electrodes, adischarge being maintained between these electrodes in the presence ofchlorine.

Titanium tetrachloride is mixed with aluminum chloride in such a waythat the heat of reaction is used as a means of raising the temperatureof the titanium tetrachloride. It may, for example, be passed into thealuminum chloride generator either separately or mixed with chlorine andmay form part of the fluidizing gas in a fluid bed reactor.Alternatively it may be mixed with the hot aluminum chloride close tothe exit from the generator. It is advantageous to heat the titaniumtetrachloride to a temperature of between about 350° C. (662° F.) andabout 400° C. (752° F.) and subsequently pass it to the aluminumchloride generator.

The injection and burning of auxiliary fuels in the reactor may beutilized to increase the temperature in the reactor and lower thepreheating temperature level requirements for the titanium tetrachloridevapors. Auxiliary fuels may be any low molecular weight organiccompounds capable of supporting combustion such as carbon monoxide,methane, propane, butane, pentane, hexane, benzene, xylene, toluene, orany combination thereof. In a preferred embodiment, propane is added tooxygen being introduced to the reactor at the first inlet point.Alternatively, the auxiliary fuel may be simply injected directly intothe reactor. In another embodiment, plasma, such as that generated by aDC arc or inductively coupled plasma, may effectively be used to heatoxygen prior to introduction into the reactor and lower the temperaturelevel requirements for the titanium tetrachloride vapors.

The proportion of oxygen which is introduced to the reactor at the firstinlet point determines to some extent the conditions within theoxidation reactor and can therefore be varied to control theseconditions. Usually at least about 50% by weight of the total oxygenfeed will be introduced at the first inlet point and preferably theproportion added at the first inlet point is from about 50% to about 95%by weight of the total oxygen feed. Most preferably the proportion isfrom about 60% to about 95% by weight. The factor determining how muchO₂ is fed to the first O₂ inlet point is determined by how much TiCl₄ isfed to the first TiCl₄ inlet. The ratio of primary TiCl₄ to primary O₂is the one that controls size. Changing the percentage of oxygen at thefirst inlet point provides control over the pigment properties to allowfor compensation for different variables. The percentage of primaryoxygen introduced at the first inlet point will depend on the desiredtint tone for the finished product. If more positive tint tones arerequired, the percentage of oxygen introduced at the first inlet pointwill decrease. Conversely, if more negative tint tones are desired, thepercentage of oxygen introduced at the first inlet point will increase.

The quantity of oxidizing gas stream introduced also depends upon theproportion of oxygen present in the gas stream. There must be sufficientoxygen to fully oxidize the total amount of titanium tetrachlorideintroduced and usually there is more oxygen than is stoichiometricallyneeded. Typically, the oxidizing gas stream will provide at least about5% by weight and preferably about 10% by weight more oxygen than isrequired to completely oxidize the titanium tetrachloride.

Aluminum chloride is present in the titanium tetrachloride to act as arutilization agent, that is, to promote the formation of rutile titaniumdioxide. Normally, the quantity of aluminum chloride used is sufficientto produce between about 0.3% and about 3.0% Al₂O₃ by weight in thetitanium dioxide product. Preferably, the amount used produces fromabout 0.3% to about 1.5% Al₂O₃ by weight in the titanium dioxideproduct. The amount of Al₂O₃ is dependent on pigment grade beingproduced. Low durability pigments use little Al₂O₃.

The process of this invention is operated at a pressure aboveatmospheric pressure. The pressure in the reactor during oxidation is atleast about 0.15 MPa above atmospheric pressure, and can range fromabout 0.15 MPa to about 4.0 MPa above atmospheric pressure. Preferably,the pressure range is from about 0.2 MPa to about 0.5 MPa aboveatmospheric pressure.

The distance between the first TiCl₄ introduction assembly and a secondTiCl₄ introduction assembly and between any further inlet points isgoverned by the rate of feed of the titanium tetrachloride and theoxidizing gas streams at the previous inlet points. Advantageously theTiCl₄ to O₂ ratio at the start of the reaction is from about 0.5:1 toabout 1.3:1. Ideally a portion of the oxygen introduced at the firstoxygen inlet point will be reacted, i.e., a sufficient amount ofparticle nucleation and rutilization has taken place, before thereactant gas stream reaches the zone of the reactor adjacent to thesecond oxygen inlet point. Hence the walls are cooled to keep fromforming hard accretions. No heat loss would likely be best. As shown inFIGS. 4-8, the second oxygen introduction assembly maybe on either sideof the second TiCl₄ inlet point and at various distances from the firstTiCl₄ inlet point without affecting the particle size of the pigment.The particle size of the pigment will not be affected provided thesecondary oxygen is introduced into a region of the reactor in which thereaction conditions are favorable for forming titanium dioxide.

Usually, the reactors used for the process of this invention have agenerally tubular shape and a portion of the oxidizing gas flow isintroduced at one end. The titanium tetrachloride inlet point is closeto the end where the oxidizing gas flow is introduced and is introducedthrough an injector of the type conventionally used for titaniumtetrachloride oxidation reactors. For example, the injector may comprisea circumferential slot in the wall of the reactor, an arrangement ofperforations in the reactor wall which may extend axially along thereactor, a single jet or nozzle, or an arrangement of jets or nozzles.

Any pipework and associated equipment used to conduct the mixture oftitanium tetrachloride and aluminum chloride from the aluminum chloridegenerator to the first inlet point will usually be formed from aceramics material to minimize corrosion. Corrosion of the reactor usedfor the process of the invention can also be reduced by constructing thefirst inlet point and the walls between the first inlet point and thesecond inlet point from a ceramics material.

Additives conventionally used in the oxidation of titanium tetrachloridecan be used in the process of this invention. For example, alkali metalsalts may be added to control the crystal size of the titanium dioxideproduced. Preferably, the alkali metal salt is a potassium salt whichcan be added as potassium chloride to the oxidizing gas stream beforethe first inlet point. The amount of potassium chloride added may befrom about 400 ppm up to about 600 ppm, but preferably the amount addedis more than about 0.5 to about 20 ppm potassium with respect to TiO₂formed. A scouring agent such as sand or titanium dioxide can also beadded to help prevent fouling of the reactor walls.

The invention provides an easily controllable process for the oxidationof titanium tetrachloride with minimum contamination of the producttitanium dioxide and without the use of inflammable liquids. Theintroduction of all the aluminum chloride with the titaniumtetrachloride added at the first inlet point generally leads to easyrutilization of the titanium dioxide formed.

The particle size of the product titanium dioxide can also be adjustedby adjusting the temperature at the first inlet point and/or thepressure in the reactor.

EXAMPLE 1

Tests were performed with cold secondary oxygen, with hot secondaryoxygen, and with plasma heated secondary oxygen.

Series 22.

This test was run with cold secondary oxygen. The base pigment producedwas Kerr-McGee Chemical Corporation (KMCC) CR 813. The raw pigment hadabout 0.5 percent Al₂O₃ and there was no potassium injection.Configurations for the oxidizer as shown in FIGS. 4, 5 and 6 weretested. The CBU of the raw pigment as a function of the primary TiCl₄ toprimary O₂ ratio is shown in FIG. 2.

Series 24.

This test series was similar to Series 22 except potassium was added atthe dual slot oxidizer (DSO) and methane was added with the secondaryTiCl₄. The results of this test are shown in FIG. 2. The two points withTiCl₄ to O₂ ratios of about 1.2 and CBUs of about −3 were obtained byadding a secondary methane flow in an attempt to improve rutilization.

Series 27.

This test was performed while producing commercial TiO₂. One bulk samplewas produced with a latex tint tone of −4.2 and a gloss of about 72 whenfinished with intense grinding. The primary TiCl₄ to primary O₂ ratioused was about 0.8 and the CBU of the sample was about −2.2. The CBU ofa sample produced with a ratio of about 1.02, but not finished was−1.42, suggesting a finished tint tone of about −4.1 or lower. Theintense milling was performed to determine whether the more positive CBUwas due to larger particles or to agglomeration. The results indicatedthe raw pigment could be ground to a stable size before finishing andthat the pigment was relatively easy to filter. This indicates the rawpigment was large particles rather than agglomerates.

Series 49.

The three previous test series indicated that the rutilization decreasedslightly with the use of cold secondary oxygen. In this test, the oxygenflow was split so that two-thirds of the O₂ was fed upstream of theprimary TiCl₄ slot and one-third was fed at the end of the cone. The DSOwas located about three feet downstream from the secondary O₂ injectionslot. The oxidizer configuration for this test is also given in FIG. 6.Two bulk samples from this test configuration and two samples from acontrol oxidizer were finished. The tint tones were −3.2 for the sampleswith secondary oxygen and about −4.2 for the control samples. All otherproperties of the finished pigments appeared to be about the same.

Series 57 and 58.

Plasma was used to heat the secondary oxygen for these tests. The mainobjective of the tests was to increase rutilization relative to thatpossible using oxygen heated with a heat exchanger. The pigment producedhad positive CBUs as in other cases using secondary O₂ with therutilization being equivalent.

The CBU of the raw pigment increased as the ratio of TiCl₄ to O₂increased at the front of the oxidizer in FIGS. 2 and 3. The slope ofthe line increases rapidly in moving from FIG. 2 through FIG. 3. Thissuggests that another variable such as the increase in production ratesor the position of the potassium injection has increased theeffectiveness of the secondary oxygen. The data in FIG. 2 was obtainedfor a KMCC CR 813 raw pigment indicating that there was no potassiuminjection, the data in FIG. 3 was obtained with potassium injected atthe DSO. The ratio of primary to secondary TiCl₄ injection (Rsp) was 0.5for the data in FIG. 2 and FIG. 3 indicating that the Rsp was not thevariable affecting the dependence of CBU on the TiCl₄ to O₂ ratio.

Test Configuration:

A description of the equipment and the basis for the design is providedbelow in the Experimental Configuration and a schematic showing theoxygen flow control is given in FIG. 1. The primary O₂ and TiCl₄ werefed to the oxidizer as is current practice. However, the primary O₂ flowwas split and a measured part of the oxygen flow sent through apreheater to second O₂ slot located immediately downstream of the secondTiCl₄ slot. The secondary O₂ flow rate was measured while the O₂ wascold and then sent to a preheater where its temperature was controlled.It was possible to control particle size using the system shownschematically in FIG. 1.

The configuration for the oxidizer is basically the same as shown inFIG. 4. FIG. 3 results indicate the configuration of the oxidizer doesnot have a major effect on the pigment properties but the DSO andsecondary oxygen spool was less affected by abrasion if the spools werefurther downstream than in FIGS. 4 or 5. Initially potassium was addedat the end of the cone but several samples were collected with potassiumadded at the DSO, particularly if rutilization was low or the CBU wasnot positive enough. The secondary oxygen preheater was installed on a6-inch line, and the control line, was also a 6-inch line. The test lineand control line were operates as near full capacity as possible.

The oxygen preheater must be capable of preheating about one half thetotal oxygen normally fed to a 6-inch oxidizer to 1038° C. (1,900° F.).An objective of the test was to determine the minimum temperature of thesecondary oxygen required for acceptable rutilization at each aluminalevel.

Test Procedures:

A detailed discussion of the test procedures is provided below in theExperimental Procedures—Test Series 62. Three sets of tests wereperformed. Each set was at a different coburned Al₂O₃ level. The lowerlevel was at approximately 0.5 percent and the higher level was at about1.2 percent coburned Al₂O₃. The third series was intermediate betweenthese levels. The primary TiCl₄ to primary oxygen ratio was varied fromthe minimum level required to protect the heat exchanger tubes and keepthe secondary oxygen slot open to a maximum TiCl₄ to O₂ ratio of about 1at the front of the oxidizer. Depending on the rate of change either twoor three intermediate samples were collected. Bulk samples werecollected from a control line, at the start of the test series and theend of the test series.

Temperatures were measured during the tests to obtain axial profilesalong the length of the tube and to obtain a radial temperature profilewith O₂ streams that were independently controlled and heated at the endof the cone for each-different TiCl₄ to O₂ ratio.

All the bulk samples from this test series were finished.

EXAMPLE 2

The primary purpose of the secondary oxygen addition was to develop amethod yielding improved raw pigment properties. The pigment particlesproduced were larger and thus the finished pigments had a more positivetint tone. The pigments produced with secondary oxygen were lessaggregated than pigment produced using the prior art oxidizerconfiguration by virtue of the fact that the pigment gets larger bycoalescing. Some aggregation was likely present as a result ofinteractions of the pigment particles on the wall of the oxidizer.Secondary oxygen does not reduce aggregation that occurs as a result ofsuch interactions. The reasons particles coalesce to a larger size withsecondary oxygen are likely because rutilization of the particles occursmore slowly and because the initial concentration of TiCl₄ is higher.Analysis of the results indicated that the main variable affecting rawpigment CBU is the TiCl₄ to O₂ ratio at the primary slot.

Test Configuration

The configuration of the oxidizer injection slots is as shown in FIG. 6.The secondary oxygen was heated in a heat exchanger consisting of aradiant section with three identical helical coils and a convectionsection at the top consisting of a number of J tubes welded together.The unit was designed to deliver 330 scfm of heated oxygen attemperatures as high as 1038° C. (1900° F.).

The temperature of the oxygen in the front of the oxidizer was higherfor the secondary oxygen tests than for normal operation because theamount of TiCl₄ per unit of oxygen is higher. This higher temperaturecame from using a greater amount of propane per unit of oxygen forsupported combustion at the sand scour nozzle. The propane to TiCl₄ratio required to reach the same mix temperature is therefore about thesame.

Test Procedures

The objective of this series of tests was to determine the effect ofdifferent TiCl₄ to O₂ ratios at the primary slot on raw pigmentproperties. The ratio of TiCl₄ to O₂ was varied from about 0.6 to about1.0 with Al₂O₃ compositions varying from about 0.5 percent to about 1.2percent. The lower value of the TiCl₄ to O₂ ratio was determined by theminimum value required to keep the secondary oxygen slot from plugging.The maximum ratio was the ratio that would not result in a decrease inparticle size or a decrease in CBU with an increase in the ratio.

The test series was divided into three subseries. The test series andmajor variables in the test were as follows:

Series 62-1.

Raw pigment with an Al₂O₃ content of 0.5 percent was produced in thistest series. The oxidizer was started at the flutter point at the startof each test, the Rsp was set at 0.2 to 0.25, and the secondary oxygenfed to the oxidizer at 927° C. (1700° F.). The first test was at theminimum TiCl₄ to O₂ ratio, the RTO, and the final test of this subserieswas at a ratio of about 1. Two or three tests were performed atintermediate ratios. Tube samples were taken to evaluate each operatingcondition. If the rutilization was below about 98.3, the amount ofpropane used for supported combustion was increased by 1 scfm. Theamount of propane was increased by up to 4 scfm until it was obviousthat increasing the amount of propane did not increase rutilization. Thesecondary O₂ temperature was then raised in 38° C. (100° F.) incrementsuntil the temperature reached 1038° C. (1900° F.) or acceptablerutilization was attained. If the percent rutile was above 99.6 percent,the TiCl₄ to O₂ ratio was increased to approximately 1.0 and if therutilization remained high the secondary oxygen temperature wasdecreased in 38° C. (100° F.) increments to determine the minimumpreheat required to attain 100 percent rutilization. Once thistemperature was determined for an RTO of 1.0, the ratio was decreasedincrementally to the minimum value described. When this sequence oftests was completed, the Rsp was increased to about 0.3 to 0.35 todetermine if conditions could be found that would produce approximately100 percent rutilization and no TiCl₄ slip.

Series 62-2.

This test series was similar to Series 62-1 except it was performedwhile producing a raw pigment with about 1.2 percent coburned Al₂O₃. Thetemperature of the secondary oxygen was set lower than 1038° C. (1900°F.).

Series 62-3.

A series of tests at an intermediate Al₂O₃ level of about 0.8 percentwas performed using the same sequence as for Series 62-1 and 62-2.

Small samples collected while the unit was operating under steadyconditions were used to determine the process variability of an oxidizerrunning with secondary O₂.

Changes may be made in the combination and arrangement of the variousparts, elements, steps and procedures described herein without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A process for the production of titanium dioxide comprising reactingtitanium tetrachloride with oxygen at atmospheric pressure or above andat a reaction temperature of at least about 700° C. in an oxidationreactor having a first reaction zone and a second reaction zone spaced adistance from the first reaction zone, the oxygen being introduced intothe reactor at a first inlet point in the first reaction zone before anytitanium tetrachloride is introduced and into at least one further inletpoint in the second reaction zone, the titanium tetrachloride introducedinto the reactor being heated to a temperature of less than about 427°C. prior to introduction, and controlling the titanium tetrachloride tooxygen ratio in the first reaction zone to thereby control the particlesize of the produced titanium dioxide.
 2. The process according to claim1 wherein the titanium tetrachloride introduced into the reactor is anadmixture with aluminum chloride.
 3. The process according to claim 2wherein the aluminum chloride is formed by reaction of aluminum andchlorine and the heat generated by this reaction is used to heat thetitanium tetrachloride introduced into the reactor.
 4. The processaccording to claim 3 wherein titanium tetrachloride is first heated to atemperature between about 350° C. and about 400° C. before being passedto an aluminum chloride generator.
 5. The process according to claim 3wherein the mixture of titanium tetrachloride and aluminum chloride isconducted to the reactor by a pipework constructed from a ceramicsmaterial.
 6. The process according to claim 1 wherein the walls of thereactor between the first inlet point and the further inlet point areconstructed from a ceramics material.
 7. The process according to claim2 wherein aluminum chloride is introduced in an amount sufficient toproduce between about 0.3 and about 3.0 percent by weight Al₂O₃ in theproduct titanium dioxide.
 8. The process according to claim 7 whereinthe amount of aluminum chloride is sufficient to produce from about 0.3to about 1.5 percent by weight Al₂O₃ in the product titanium dioxide. 9.The process according to claim 1 wherein the titanium tetrachloride isintroduced into the reactor at a temperature of about 399° C.
 10. Theprocess according to claim 2 wherein the aluminum chloride is mixed withtitanium tetrachloride prior to introduction into the reactor.
 11. Theprocess according to claim 1 wherein oxygen introduced at the firstinlet point is preheated to a temperature between about 815° C. andabout 982° C.
 12. The process according to claim 11 wherein said oxygenintroduced at the first inlet point is preheated to a temperature ofabout 954° C.
 13. The process according to claim 1 wherein oxygenintroduced at the further inlet point is heated to a temperature betweenabout 25° C. and about 1037° C. and is introduced in an amountsufficient to react with unreacted titanium tetrachloride.
 14. Theprocess according to claim 13 wherein at least about 50 percent byweight of the oxygen is introduced at the first inlet point.
 15. Theprocess according to claim 14 wherein from about 50 to about 95 percentby weight of the oxygen is introduced at the first inlet point.
 16. Theprocess according to claim 15 wherein from about 60 to about 95 percentby weight of the oxygen is introduced at the first inlet point.
 17. Theprocess according to claim 1 wherein the amount of oxygen introduced isequivalent to at least about 5 percent by weight more than is requiredto completely oxidize the titanium tetrachloride.
 18. The processaccording to claim 17 wherein the amount of oxygen introduced isequivalent to at least about 10 percent by weight more than is requiredto completely oxidize the titanium tetrachloride.
 19. The processaccording to claim 1 wherein the pressure in the reactor is betweenabout 0.15 MPa and about 4.0 MPa above atmospheric pressure.
 20. Theprocess according to claim 1 wherein an auxiliary fuel is added to theoxygen being introduced to the reactor at the first inlet point.
 21. Theprocess according to claim 20 wherein the auxiliary fuel is carbonmonoxide, methane, propane, butane, pentane, hexane, benzene, xylene,toluene, or combinations thereof.
 22. The process according to claim 1wherein the oxygen being introduced to the reactor at the first inletpoint is heated with plasma.
 23. The process according to claim 1wherein a potassium salt is added to oxygen introduced at the firstinlet point before mixing with the titanium tetrachloride, the potassiumsalt being added in an amount equivalent to form about 400 parts permillion to about 600 parts per million potassium chloride by weight withrespect to the titanium dioxide product.
 24. The process according toclaim 23 wherein the amount of potassium is equivalent to more thanabout 20 parts per million by weight with respect of titanium dioxideproduct.
 25. The process according to claim 1 wherein the walls of thereactor are cooled.
 26. The process according to claim 25 wherein thewalls of the reactor are cooled by providing a purge of nitrogen orchlorine gas.
 27. The process according to claim 1 wherein at least oneof the inlet points comprises a circumferential slot in the wall of thereactor.
 28. A process for the production of titanium dioxide in anoxidation reactor having a first reaction zone and a second reactionzone spaced a distance from the first reaction zone, the processcomprising: introducing an amount of oxygen in the first reaction zone;introducing an amount of titanium tetrachloride in the first reactionzone wherein the amount of oxygen introduced in the first reaction zoneand the amount of titanium tetrachloride introduced in the firstreaction zone defines a titanium tetrachloride to oxygen ratio; reactingthe titanium tetrachloride with the oxygen at a reaction temperature ofat least about 700° C. to produce titanium dioxide; controlling thetitanium tetrachloride to oxygen ratio in the first reaction zone tothereby control the particle size of the produced titanium dioxide; andintroducing secondary oxygen in the second reaction zone in an amountsufficient to react with unreacted titanium tetrachloride therein. 29.The process according to claim 28 further comprising admixing thetitanium tetrachloride introduced into the reactor with aluminumchloride before introducing the titanium tetrachloride into the reactor.30. The process according to claim 29 further comprising: generating thealuminum chloride by reacting aluminum with chlorine in an aluminumchloride generator; and heating the titanium tetrachloride introducedinto the first reaction zone with heat generated from the aluminumchloride generator.
 31. The process according to claim 30 furthercomprising preheating the titanium tetrachloride to a temperaturebetween about 350° C. and about 400° C. before passing the titaniumtetrachloride to the aluminum chloride generator.
 32. The processaccording to claim 29 wherein the aluminum chloride is introduced in anamount sufficient to produce between about 0.3 and about 3.0 percent byweight Al₂O₃ in the titanium dioxide product.
 33. The process accordingto claim 28 further comprising introducing a second addition of titaniumtetrachloride into the reactor near the second reaction zone.
 34. Theprocess according to claim 33 further comprising admixing the secondaddition of titanium tetrachloride introduced into the reactor withaluminum chloride before introducing the titanium tetrachloride into thereactor.
 35. The process according to claim 34 further comprising:generating the aluminum chloride by reacting aluminum with chlorine inan aluminum chloride generator; and heating the second addition oftitanium tetrachloride introduced into the reactor with heat generatedfrom the aluminum chloride generator.
 36. The process according to claim35 further comprising admixing the titanium tetrachloride introducedinto the first reaction zone with the aluminum chloride beforeintroducing the titanium tetrachloride into the first reaction zone. 37.The process according to claim 36 further comprising heating thetitanium tetrachloride introduced into the first reaction zone with theheat generated from the aluminum chloride generator.
 38. The processaccording to claim 28 wherein the titanium tetrachloride is introducedinto the reactor at a temperature of about 399° C.
 39. The processaccording to claim 37 wherein the second addition of titaniumtetrachloride is introduced into the reactor at a temperature of about399° C.
 40. The process according to claim 37 wherein the titaniumtetrachloride introduced into the first reaction zone and the secondaddition of titanium tetrachloride is introduced into the reactor at atemperature of about 399° C.
 41. The process according to claim 28further comprising preheating the oxygen introduced in the firstreaction zone to a temperature between about 815° C. and about 982° C.42. The process according to claim 28 further comprising preheating thesecondary oxygen introduced in the second reaction zone to a temperaturebetween about 25° C. and about 1037° C.
 43. The process according toclaim 41 wherein from about 50 to about 95 percent by weight of theoxygen introduced in the reactor is introduced in the first reactionzone.
 44. The process according to claim 28 wherein the amount of oxygenintroduced in the first reaction zone and the second reaction zone isequivalent to at least about 5 percent by weight more than is requiredto completely oxidize the amount of titanium tetrachloride introduced inthe reactor.
 45. The process according to claim 28 further comprisingoperating the reactor at a pressure between about 0.15 MPa and about 4.0MPa above atmospheric pressure.
 46. The process according to claim 28further comprising adding an auxiliary fuel to the oxygen beingintroduced in the first reaction zone.
 47. The process according toclaim 46 wherein the auxiliary fuel is carbon monoxide, methane,propane, butane, pentane, hexane, benzene, xylene, toluene, orcombinations thereof.
 48. The process according to claim 28 furthercomprising heating the oxygen being introduced in the first reactionzone with plasma.
 49. The process according to claim 28 furthercomprising heating the secondary oxygen being introduced in the reactorwith plasma.
 50. A method for producing titanium dioxide particles in atleast two reaction zones, comprising: (a) introducing titaniumtetrachloride in a first reaction zone; (b) introducing oxygen into thefirst reaction zone to oxidize titanium tetrachloride therein and formtitanium dioxide; and (c) oxidizing unreacted titanium tetrachloridefrom the first reaction zone in a second reaction zone by reaction withoxygen to produce additional titanium dioxide; wherein the reaction ofoxygen and titanium tetrachloride takes place at above atmosphericpressure and at least about 700 degrees Celsius, and the size of thetitanium dioxide particles produced through the method is controlled bycontrolling the ratio of titanium tetrachloride to oxygen in the firstreaction zone.
 51. The method of claim 50, further comprisingintroducing additional titanium tetrachloride into the second reactionzone and oxidizing the additional titanium tetrachloride in the secondreaction zone by reaction with oxygen to form titanium dioxide.
 52. Themethod of claim 50, wherein the titanium tetrachloride introduced in thefirst reaction zone is an admixture with aluminum chloride.
 53. Themethod of claim 52, wherein the aluminum chloride is formed by reactionof aluminum and chlorine and the heat generated by this reaction is usedto heat the titanium tetrachloride introduced into the first reactionzone.
 54. The method of claim 53, wherein the titanium tetrachloride isfirst heated to a temperature between about 350 degrees Celsius andabout 400 degrees Celsius before being passed to an aluminum chloridegenerator.
 55. The method of claim 51, wherein the titaniumtetrachloride introduced in the second reaction zone is an admixturewith aluminum chloride.